music keyboard

i plan on getting a new imac this year after leopard comes out and a design or system change. that being said, i want to get a keyboard for my wife so she can learn to use it and hook it up to the mac to record whatever she may want to record. what are some of the things i should look for on a music keyboard to make sure it is mac compatible ? also any suggestions on a keyboard are welcomed. i would like to stay below $200.